The present invention relates to video signal processing and, in particular, to a method and arrangement for reducing the bandwidth and/or time required to transmit continuous-tone images which have been processed for presentation on bi-level display media.
A bi-level display medium is characterized by a plurality of closely spaced display points, or cells, each of which can have one of two visual states -- fully energized (on) or fully de-energized (off). Plasma panels and pin printers, for example, are common types of bi-level display media. Picture images and other graphic data are readily displayed on a bi-level display medium via selective energization of its cells. However, due to their bi-level nature, such display media, unlike cathode-ray tube arrangements, for example, are not capable of representing gray scale, i.e., a range of intensities, at each display point location. Nonetheless, the gray-scale information in an image can, advantageously, be approximated on a bi-level display medium by known thresholding techniques. The image to be presented is scanned in a row-and-column matrix of picture elements each corresponding to a cell of the display medium. The intensity of each picture element is quantized into one of, for example, 256 levels. A threshold value is assigned to each picture element in accordance with a predetermined criterion. A cell of the display medium is energized only if the intensity of the corresponding picture element exceeds the threshold value assigned thereto.
A number of thresholding techniques of the above-described type are known in the art, each having a different criterion for assigning appropriate threshold values to the scanned picture elements. For example, so-called adaptive threshold techniques assign threshold values in response to the intensities of selected picture elements in the scanned image. The constrained average technique disclosed in the copending patent application of J. F. Jarvis, Ser. No. 576,163, filed May 9, 1975, now U.S. Pat. No. 3,961,134 and assigned to the same assignee as this application, is exemplary.
So-called "non-adaptive" thresholding techniques, on the other hand, assign threshold values to each picture element solely in accordance with the position of that picture element in the picture element matrix. Foremost among the nonadaptive thresholding techniques is the ordered dither, or more simply "dither," technique. In a dithered display system, the picture elements are divided into a plurality of submatrices. Each picture element from each submatrix is assigned a spatially corresponding threshold value from a predetermined "dither matrix." A display cell is energized as described above, i.e., only if the intensity of the corresponding picture element exceeds the threshold value assigned to that picture element.
Since each cell of a bi-level display medium can be in only one of two states, its intensity can be specified by a single "display bit." By contrast, eight bits are required to specify in which of the 256 intensity levels each picture element of the original scanned image resides. The number of bits per picture element, or bit rate, determines the bandwidth per unit time or, equivalently, the time per unit bandwidth required to transmit a frame of video information. Thus an image processed via one of the above-described thresholding techniques can be transmitted at substantially reduced bandwidth and/or transmission time as compared to a full-gray-scale, e.g., standard television, image. (The tradeoff is slightly reduced spatial resolution and some loss of gray-scale information.) However, for many video systems it may be desired to reduce bandwidth and/or transmission time requirements even further since this almost invariably translates into lower capital and operating costs. Moreover, it appears that the economic feasibility of such still-experimental video systems as electronic mail and other fast facsimile arrangements will hinge to a great extent on the degree to which the bandwidth and/or transmission time required by such systems can be minimized.